cannon



Nov. 13, 1962 w. D. CANNON DELAY AND AMPLITUDE CORRECTIVE SYSTEM 2Sheets-Sheet 1 Original Filed Aug. 12, 1958 m 0; OF I ATTORNEY 2Sheets-Sheet 2 Original Filed Aug. 12, 1958 U R x .1 m \g m .wm ufl mm 0mm I M an NmW mZ 2 0w ON 9-. I l l l l I l l l l I I I l I l l I l I I lI I l l l lllll IVY" ATTORNEY United States Patent Office Re. 25,285Reissued Nov. 13, 1962 25 285 DELAY AND AMPLITUIlE CORRECTIVE SYSTEMWilliam D. Cannon, Metuchen, NJ., assignor to The Western UnionTelegraph Company, New York, N.Y., a corporation of New York OriginalNo. 2,966,633, dated Dec. 27, 1960, Ser. No. 754,619, Aug. 12, 1958.Application for reissue Dec. 14, 1961, Ser. No. 160,420

11 Claims. (Cl. 330-20) Matter enclosed in heavy brackets II appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

This invention is concerned with corrective networks for insertion in acommunications line, and more particularly with active networks for usein correcting delay distortion in facsimile transmission lines.

The advantages of using corrective networks of the active rather thanthe passive type are described in my US. application No. 621,476, filedNovember 9, 1956 for Method and Means to Overcome Delay Distortion,wherein it is disclosed that by the employment of a network comprising aplurality of active circuits connected in cascade, it becomes possibleto provide independent adjustments for delay and frequency correctingcomponents, which are practical only because of their adaptability toadjustment with the aid of an improved measuring instrument theredescribed.

In the instant invention, a new mode of operation of the active networkelements is found to preserve the advantages stated in theaforementioned application, while at the same time giving rise tosignificant new advantages of reliability, space, weight, and powerreduction as hereinafter described.

Operation of the active elements of the said net-' works as currentamplifiers to drive furtherimpedance transforming active elementsresults in an exceedingly compact assembly. Transistors whichwarecurrently available can be employed, by the aid of the special circuitrydisclosed, to fulfill the theoretical requirements for matching anddriving an amplitude and delay corrective network with higheffectiveness. 3

It is accordingly an object of the present invention to provide acompact equipment for correcting delay distortion in a line by means ofsimple adjustments.

It is a further object to provide such means of correction, adaptable torapid adjustment toa line, in conjunction with visual distortiondisplaying equipment.

Another object of the invention is to provide increased reliability insuchadevice.

A still further object is to provide such an active cor: rective deviceadapted for receiving its operating power over a communication channel,by reason of a greatly reduced power consumption.

Other objects and advantages of the present invention will appear from aconsideration of the following detailed description of a specificillustrative embodiment of the present invention and the drawingswherein:

FIG. 1 is an electrical circuit diagram of an initial portion of a delayand amplitude corrective system according to the present invention; and

FIG. 2 is an electrical circuit diagram of a remaining portion of suchsystem.

Referring now to the drawings, in FIG. 1 is seen an attenuator 1 havinga pair of wires for input connection from a signal channel not shown,whose delay characteristics are desired to be corrected. The attenuatorshown is of the well known H pad type as used in abalanced line, but mayequally well be one of the equivalent arrangements, such as an L padwhen other types of signal channel are to be accommodated. This isconveniently arranged by making the attenuator 1 of plug-inconstruction, whereby a variety of different attenuator types and valuescan be made available for the correction network, which can then beidentically reproduced in quantity without need for including a levelsetting. variable attenuator. The output of attenuator 1 is ap plied toinput transformer 2 which conveniently has a turns ratio of 3:1 and aninterwinding shield which is grounded as shown. Emerging in unbalancedconfiguraw tion with one side grounded, the signal current is ap pliedthrough a resistor 3 which, because of its control. of the signalcurrent, provides a degree of automatic volume control and establishesthe forward bias current on the emitter 4 of transistor 5, to which itis connected. Transistor 5, like all of the other transistors in theinstant invention, which are identical in type and size, is of theso-called p-n-p high gain audio, low power germanium, grown punctiontype. Many other types of transistor will also perform satisfactorily,however, since the instant invention is very stable and non-critical inoperation. Base lead 6 is connected to the junction of resistors 7 and 8which form a divider from volt supply line 9 to positive ground, thusestablishing a suit-'- able supply potential for the circuit ofcollector 11.

Resistor 12 provides a forward bias current to the Y emitter 4corresponding to a desired value of transistor current gain. Thecollector 11 outputcircuit of transistor 5 operates at a very muchhigher impedance level than the above described input circuit, and thisis found. to be especially advantageous for the operation of the 1distortion correcting network connected thereto. The

said distortion correction network comprises the transformer 13, and avoltage divider connected acros the input terminals thereof andcomprising resistors 14 and 15 connected in series and having theircommon junction connected to the 25 volt supply line 9.- In addi-- tionthereto, the network includes a variable resistor 16 connected betweencollector 11 and the resistor 14 asone variable leg of the correctingnetwork, and parallel: connection of variable resistor 17, condenser 18,and inductor 19 as a second such leg connected between collector 11 andresistor 15. The ratio of-the values of resistors 14 and 15 is critical,and must be maintained at. substantially about 3:1 in order for thecircuit to operate. successfully. v 2

This is under the preferred, practical, and described I condition thatadjustable resistors 16 and 17 have equal values at median gain. ,It ispossible to provide an operative circuit wherein said resistors 16 and17 do not have such equal values, provided that the ratio of the values,

- of resistors 14 and 15 be then established at a value de parting fromthe aforesaid 3:1 ratio by an amount deter: mined by the degree to whichresistors 16 and 17 depart from equality. No advantage results from sucha design, however, and the practical convenience of employing identicalcomponent for resistors 16 and 17 is thereby lost. The interdependenceof all of the said resistors is expressed by the following formula:

wherein R etc. represent the respective resistors 15, etc;

as above mentioned, and from which it can be seen that for smalldeviations from the aforementioned preferredv design, a condition ofsubstantial proportionality exists between the expressed ratios, alsoexpressible as a condi tion of equal percent of change thereof.

The aforesaid signal passband upon which the correction supplied by thiscircuit is to operate.

The frequency range over which each such distortion corrective networkis operative to provide correction, conveniently may be 375 cycles in apractical installation, in which case twenty-five successive sectionswould be employed to obtain the best performance available from a linebadly degraded by delay distortion, over the facsimile frequency band of600-1 5,000 cycles per second. The successive sections are spaced atapproximately uniform frequency intervals and are operative not onlywithin their assigned interval but in diminishing degree for asubstantial distance above and below the frequencies thereof.Essentially uniform spacing of the networks according to frequencyrather than on a percentage basis is permissible because the higherfrequencies of the facsimile band are of greater importance in producinga facsimile image of high quality than are the lower frequencies whichare more remote from the carrier frequency. A useful refinement of suchspacing, however, is to group the networks somewhat more closelytogether frequency-wise near the center of the facsimile band then atits ends. By this means, greater distortion corrective power of thenetworks is concentrated in the regionusually having the greatestdistortion. It should be understood, however, that in cases where thedistortion to be corrected is less severe than above described, asmaller number of corrective networks, more broadly tuned and morewidely spaced in the facsimile band, are sufiicient to provide anadequately corrected signal. Successive identical circuits tuned to asuccession of spaced frequencies by preselection of the appropriatevalues of condenser 18 and inductor 19 are employed to providecorrection to adjacent narrow frequency bands throughout the spectrum ofinterest although the use of only a few or even of a single such filtersection is entirely feasible where only a small degree of correction isneeded. Variable resistors 16 and 17 are adjusted simultaneously andidentically to control the delay, and resistor 17 alone to control theamplitude, of the assigned frequency band for the particular pair ofresistors involved, substantially Without interaction of the two effectsand these comprise the adjustments necessary to be made in each sectionof the corrective system for the several bands. Such adjustments areconveniently made by means of a visual display of the delaycharacteristics of the entire line under consideration together with thecorrective network in question, as can be accomplished with the aid ofthe equipment and technique disclosed in my application Serial No.621,476, filed November 9, 956, and entitled Method and Means toOvercome Delay Distortion."

Emerging from transformer 13 which is a step-down transformer formatching the high impedance corrective network above described to theinput impedance of the following transistor, signals are applied througha resistor '21 to the base 22 of transistor 23 which is arranged in thecommon emitter connected configuration for maintaining signal amplitude,the signal circuit being completed through emitter 24, resistor 25 andresistor 26. Forward bias current is provided through resistor 27connected to the negative supply line. A resistor 20' connected betweenbase and collector of transistor 23 provides stabilizing negativefeedback to the operation of said transistor. The directly coupledfollowing transistor 28 is similarly supplied with bias current bydivider resistors 29, 30, the same current flowing through its emitter31, collector 32 and the emitter 24 of transistor 23, and resistor 25,for modulation by the transistor 23 of the bias of the base connectedimpedance matching transistor 28'. It is to be noted that in the case ofthe transistors just described, and in the case of all other transistorsin the network of this invention, a single source of direct current isall that is needed to energize them, and this makes possible the use ofa simple and convenient power supply later to be described.

' and tuned to operate over successive and adjacent narrow frequencybands within the desired pass band of the channel.

Numeral 39' denotes a final corrective unit comprising adjustable delayand amplitude corrective circuitry as above described and connected toenergize the primary winding of the interstage transformer 41 having apushpull output. The secondary center tap of transformer 41 is seen tobe provided with a bias potential by connection to the junction ofvoltage divider resistors 42 and 43 which are connected between groundand the 25 volt negative supply line, and bias current, limited by baseresistors 44 is thus supplied to the two pairs of transistors 45, whichare connected in a parallel push-pull circuit for the purpose ofsupplying substantial signal output power from the system. This outputstage amplifies the entire facsimile band with equal gain, and is forthe purpose of'restoring the signals to line level, from the level ofapproximately zero decibels which prevails in the corrective networksfor reasons of convenience.

Such output is supplied to a suitable balanced load circuit by means ofa transformer 47.

A power supply to actuate the active corrective networks in the abovedescribed system is conveniently composed of a step-down transformer 51,having a fuse 52 in the input wires connecting it to the power mains,and'having its low voltage secondary winding connected to the diodebridge rectifier 53 composed of identical semiconductor rectifiers 54.Full wave rectified current is withdrawn from the alternate vertices ofbridge rectifier 53 and converted to substantially pure direct currentby a single section filter section consisting of an input condenser 55which smooths and increases the voltage, a circuit anti-resonant atdouble the line frequency consisting of choke 56 and condenser 57 andcomprising a relatively high impedance to the flow of current at ripplefrequency, and the condenser 58. Direct current output is providedbetween positive ground and a supply line of minus 25 volts, asindicated on the drawing. It is because of the very small current drawnby the active components of the instant system in comparison with vacuumtube operated devices of the prior art that it is practical to use asingle section filter in the power supply as described, since the lowcurrent require-- ments, which in practice amount to only about eight.milliamperes, enables an inductor 56 to be used which has a sufficientlyhigh ratio of reactance to resistance in order to enable it to be tunedto an efiective peak. of anti-resonance by means of the condenser 57. Afurther important advantage of the instant invention is gained by reasonof the fact that the power required to operate the instant system isonly about two watts, instead of about watts, as required byarrangements of the prior art. As a consequence of this,v several of thecomponent parts of the power supply are not only smaller, but the entireassembly of corrective networks and power supply can be housed in asmaller container than would otherwise be possible, since little or noprovision need be made for dissipation of waste heat by means ofextended radiating surfaces, air convection, or other means. Itisifurther practical with the aid of the instant invention, because ofthe fact the low power consumption attained does not represent anexcessive loading for communications circuits, to energize the entirecorrective system by electricity transmitted over the signal channel ata convenient frequency outside the communication band, and to .do thisdespite.

the relatively low power handling ability usually encountered in suchsignal channels.

Although this invention has been described in terms of a specificillustrative example of the best embodiment thereof, variousmodifications will occur to those skilled in the art, which do notdepart from the essential spirit of the invention disclosed. It istherefore intended that this invention shall be limited only by theappended claims.

What is claimed is:

1. Transmission delay and amplitude distortion corrective meanscomprising as serially connected transmission elements, input linecoupling means, at least one corrective bridge section, andinter-section matching means connected between each said correctivebridge section and a next successive said corrective bridge section andterminal output [amplifying] means wherein: said inter-section matchingmeans comprises a first transistor in common emitter configuration, asecond transistor in common base configuration being connected to bedirectly driven [connected for driving] from the collector of said firsttransistor, and a first resistor interconnecting the base of the firstand the emitter of said second transistor, said terminal output[amplifying] means cornprises a transistor [amplifier], and each saidcorrective bridge section comprises two separate shunt paths across aninput, a first said path comprising [adjustably] a damped antiresonantcircuit means [joined] in series with a first [ratio] impedance througha first joint therebetween, and a second said path comprising [anadjustable] a second resistor [joined] in series with a second [ratio]impedance through a second joint therebetween, the ratio of said firstand second [ratio] impedances being substantially about 3:1, and anoutput means [including said ratio impedance for coupling out a bridgeoutput from] coupled across [the] said first and second joints, said[adjustable] second resistor having a value substantially about equal tothe equivalent parallel damping resistance of said damped antiresonantcircuit means, and said dumped antiresonant circuit means of each ofsaid corrective bridge sections [means] being antiresonant in theneighborhood of a specific preselected segment of transmission frequencyband to be corrected, whereby simultaneous and equal adjustment of thedegree of damping of said damped antiresonant circuit means and of thevalue of said [adjustable] second resistor influences only delaycorrection, and adjustment of said [adjustable] second resistor valuealone influences only amplitude correction, of a said specific bandsegment.

2. The device of claim 1 wherein said [adjustably] damped antiresonantcircuit means comprises an adjustable damping resistor.

3. The device of claim 2 [wherein said] comprising input line couplingmeans [comprises] having an attenuator, a coupling transformer, and acommon base connected transistor amplifier operated from saidtransformer.

4. The device of claim 1 wherein said [adjustably] damped antiresonantcircuit means comprises a parallel resonant circuit and an adjustabledamping resistor connected thereacross.

5. The device of claim 4 wherein said [ratio] first and secondimpedances comprises a pair of resistors, and [the] said means forcoupling out a bridge output comprises a transformer having a [primary]winding connected across [the] said first and second joints.

6. The device of claim 4 [wherein said] comprising terminal outputamplifying means [comprises] having 7 a plurality of parallel pairs ofcommon emitter connected transistors, connected for push-pull operationbetween the pairs, and wherein an output transformer is connected to theoutput of said transistors.

7. A device [which includes corrective means] according to claim 6,comprising a power supply therefor, and a filter choke in said powersupply which is tuned to resonance at the second harmonic of the powerline frequency.

8. Transmission delay and amplitude distortion cor rective meanscomprising as serially connected transmission elements, a correctivebridge section, and matching means fed by said corrective bridge sectionwherein: said matching means comprises a first transistor in commonemitter configuration, a second transistor in common base configurationdriven from the collector of said first first transistor, and a firstresistor interconnecting the base of the first and the emitter of thesecond transistor, and said corrective bridge section comprises twoseparate shunt paths across an input, a first said path comprisingdamped antiresonant circuit means having a first resistor in series witha second resistor through a first joint therebetween, and a second saidpath comprising a third resistor in series with a fourth resistorthrough a second joint therebclween, said first, second, third andfourth resistors having values varying in relation to R 2 R where R issaid first resistor, R is said second resistor, R is said third resistorand R is said fourth resistor, and means including said first and secondresistors coupled to said first and second joints to obtain the outputsignal of said corrective bridge section from across said first andsecond joints, and said damped antiresonant circuit means of saidcorrective bridge section being antiresonant at a specific preselectedsegment of the transmission frequency band to be corrccted, wherebysimultaneous and equal adjustment of the degree of dumping of saiddamped antiresonant circuit means and of the value of said secondresistor influence only delay correction, and adjustment of said secondresistor value alone influences only amplitude correction, of saidspecific preselected band segment.

9. The device of claim 8 wherein said damped antiresonant circuit meanscomprises a parallel resonant circuiz and an adjustable damping resistorconnected thereacross.

10. The device of claim 9 wherein said means to obtain the output signalof said corrective bridge section comprises a transformer having awinding connected across said first and second joints.

1]. The device of claim 10 comprising a power supply therefor, and afilter choke in said power supply tuned to resonate at the secondharmonic of the power line frequency.

References Cited in the file of this patent or the origlnal patentUNITED STATES PATENTS 2,710,315 Tongue June 7, 1955 2,790,035 I-Iylos eta1 Apr. 23, 1957 2,889,416 Shea June 2, 1959 2,889,453 Stern June 2,1959

